1. Field of the Invention
This invention relates to electrical devices and assemblies and methods of making such devices and assemblies.
2. Introduction to the Invention
Circuit protection devices comprising a conductive polymer composition having a positive temperature coefficient (PTC) are well-known. Such devices which are intended for surface mounting onto a substrate, e.g. a printed circuit board, are disclosed in U.S. Pat. No. 5,831,510 (Zhang et al), U.S. Pat. No. 5,852,397 (Chan et al), and U.S. Pat. No. 5,864,281 (Zhang et al), and International Publications Nos. 94/01876 (Raychem Corporation) and 95/08176 (Raychem Corporation), and commonly assigned application Ser. No. 09/181,028 (Graves et al, filed Oct. 27, 1998), now U.S. Pat. No. 6,651,315, issued Nov. 25, 2003, the disclosures of which are incorporated herein by reference. Such circuit protection devices generally comprise first and second laminar electrodes; a laminar PTC resistive element sandwiched between the electrodes; a third (residual) laminar conductive member which is secured to the same face of the PTC element as the second electrode but is separated therefrom; and a cross-conductor which passes through an aperture in the PTC element and connects the third conductive member and the first electrode. This permits connection to both electrodes from the same side of the device, so that the device can be connected flat on a printed circuit board, with the first electrode on top, without any need for leads. The resistive element preferably comprises a laminar element composed of a PTC conductive polymer. Preferably the device comprises an additional conductive member and an additional cross-conductor, so that the device is symmetrical and can be placed either way up on a circuit board.
When two of these devices are physically secured together in a stacked configuration, a composite device can be formed. Such composite devices have the same small “foot-print” on the board, i.e. occupy a small area, as a single device, but they have a lower resistance than can be conveniently produced by using a single device. In addition, the power dissipation of such a composite device is not substantially different from the power dissipation of one of the devices alone. As a result, the composite device has a lower resistance for a given hold current, where “hold current” is the largest current which can be passed through a device without causing it to trip.